Five things to consider when setting up a subwoofer in your car

Understanding how to get the bass you want

JL Audio's 10" (left) and 12" (right) W0v3 subwoofers. The size and number of subs you choose will largely depend on the amount of room you have available.

When it comes to picking the right sub for your system and setting it up to deliver exactly the kind of bass reproduction you want, we recommend that you consider five elements:

sub size

enclosure type

power

impedance

low-pass filters

Here’s a quick look at how all of these elements work together.

Subwoofer size:
how big should it be?

Really, the size of sub you choose is mostly a pragmatic matter — it depends primarily on the amount of space available in your vehicle. If you’re looking for maximum slam, consider the largest sub that your space allows. But if your space is limited, or you want to maximize your remaining cargo space, even adding a small sub, such as an 8" model, can deliver forceful, deep, and satisfying results.

In terms of performance, generally speaking, a subwoofer with a larger cone is capable of playing deeper bass than one with a smaller cone, simply by virtue of being able to move more air. How noticeable that difference is, however, is a matter of debate, and really depends on the other four factors we’ll talk about next. That’s why you should feel comfortable letting your available space dictate your sub size.

Enclosure type: which should you choose?

A sealed enclosure will provide an airtight home for your subwoofer, for tight, natural-sounding bass.

Every sub needs an enclosure in order to perform, and the type of enclosure (also simply called a box) you choose for yours will have the biggest impact on the kind of bass reproduction you get. The two most common types of enclosure are sealed and ported. Sealed enclosures are airtight, while ported enclosures feature a vent or a tube that allows air to pass in and out of the box.

A ported enclosure features vents (at the bottom of this box) that let air move in and out of the box to reinforce the sub's output for forceful, resonant bass.

When trying to describe the difference between the sounds of sealed and ported boxes, you’ll often hear that sealed boxes are for "tight and accurate" bass, while ported enclosures are "loud and boomy." While this is useful as a shorthand explanation, it doesn’t quite fully capture what each box does and how it does it.

Sealed enclosures

With sealed boxes, what is often referred to as "tight" bass is the result of what’s called linear, or flat, bass response. This means that across the bass spectrum, the output of a sub in a sealed box will gradually and evenly decline as the bass notes get lower, without any excessive spikes or dips. Fans of music such as classical or jazz will get a lot out of a sub in a sealed enclosure, as the linear response ensures that instruments like the cello or the low keys of the piano will sound clean and natural.

Ported enclosures

Ported boxes are great for people who want to get lots of low-frequency output. With a ported box, when the sub moves forward it creates a bass note. Then, when it moves backwards, it forces air out of the box’s port, which reinforces the bass output and reduces air pressure inside the box itself, allowing the cone to move more freely. This means you’ll get more output from a ported box than you will from a sealed box receiving the same amount of power — that's why ported subs are often described as "louder."

Using pink noise, we tested two 12" JL Audio subs in the Crutchfield Labs: one in a sealed box, the other in a ported. The sub in the sealed box showed a natural response that was smooth and linear. The ported box played efficiently and, thanks to its 34Hz tuning frequency, reinforced the sub's output in the lower end.

The "boomy" aspect of a ported box is a little trickier to understand. The port of a given box will have a specific "tuning frequency" (you can find this in the "Details" tab of a given box's web page when you’re shopping with us), which describes the frequency at which air naturally resonates through the port.

Air coming out of the port will reinforce the bass that the sub is playing. The closer the notes are to the tuning frequency, the greater the reinforcement — the "boom" that people describe. Below the tuning frequency, the sub’s output drops off more dramatically than it does in a sealed box, so the lower a box’s tuning frequency is (and most are in the 30s), the deeper a sub in a ported box will comfortably play.

Ported boxes are popular with fans of rock, who like the chest-punch of the kick drum that efficient ported subs can provide, and of rap, hip-hop, and electronic dance music, for whom the low-end reinforcement adds the resonance they crave.

Which one is for me?

For many, the choice between sealed and ported boxes comes down to a combination of power, size, and output considerations.

A sealed box will be smaller than a comparable ported box, but require more power to achieve comparable output because of the strength of the air pressure inside the box.

Conversely, while ported boxes deliver greater output from comparable amounts of power, they tend to be larger and will take up more space in your cargo area.

As you’re shopping, you’ll also run across boxes called "bandpass enclosures." These remarkably efficient enclosures combine sealed and ported designs to deliver very high output over a specific band of low frequencies. To learn more about them, and get more info on sealed and ported boxes, check out our subwoofer enclosure article.

Power: more is better

The sound waves created by bass notes are big. Very big. And subwoofers require lots of power to generate them — the kind of power that only an external amplifier can provide.

When you’re looking for an amp to power your sub, there are two specs you’ll want to pay most attention to: the sub’s upper RMS power-handling spec, and the amp’s RMS output (RMS refers to the amount of power a speaker can continuously handle, or an amp can continuously put out; maximum power specs refer only to short bursts and are irrelevant for sub/amp matching).

With power, the easy rule to remember is that more is better. In fact, you can do more damage to a sub by underpowering it than by sending it a bit too much power. (Think of a car trying to climb a steep hill. If it has a big V-8 engine, it'll handle the hill with no problems. If it has a small, 3-cylinder motor, it'll struggle, and likely overheat.)

Ideally, then, your amp’s RMS output wattage will be 75% to 150% of your sub’s top RMS power-handling specification. And 150% will be better than 75%. You can learn more in our article on matching subs and amps.

Impedance: wiring multiple subs, or subs with dual voice coils

This diagram shows a sub with dual voice coils wired to present 4 ohms safely to a mono amplifier. You can learn more by checking out other subwoofer wiring diagrams.

All electrical circuits and components, such as the voice coils of a subwoofer, offer some natural resistance to the flow of electricity. This resistance is called impedance, and is measured in ohms (and expressed with the Greek letter omega: Ω).

When you’re wiring your sub to your amp, you need to make sure that their impedance specs match (in the case of the amp, this is called being "stable" when presented with a particular impedance rating). For example, if you have a sub with an impedance rating of 2 ohms, you need to make sure your amp is 2-ohm stable.

It’s not unusual for audio enthusiasts to drive more than one sub with a single amplifier. There are ways to wire multiple subs so that the impedance they present to an amp matches the amp’s impedance stability specs. Similarly, some subwoofers feature two voice coils, which can be wired together to present an acceptable impedance to an amp.

The lower the impedance a sub presents to an amp, the more power the amp will send to the sub. That’s why it’s so important to make sure that you present an acceptable impedance to your amp: if the impedance is too low, the amp will overheat trying to supply more power than it’s capable of delivering. On the other hand, when you’re wiring multiple subs, or dual voice coils, presenting the smallest acceptable (safe) impedance to your amplifier is a great way to maximize power output and get the best possible subwoofer performance.

Low-pass and subsonic filters

Low-pass filters keep high notes away from your sub, helping maximize its performance, and letting you blend its sound with your full-range speakers.

Just about every subwoofer amp on the market these days provides a low-pass filter. This helps ensure that only the low frequencies in your music reach your sub. The filter lets you choose the frequency at which it begins to block the high notes, which helps you blend your sub’s output naturally with the rest of your full-range speakers.

Setting your filter typically consists of lots of experimentation — there’s no one, right "formula" for where to set it.

You’ll want to make sure that your filter isn't set so high that the sub overlaps too much with the woofers of your full-range speakers, because this can result in overemphasis on one range of frequencies (say, around 120 Hz or so; think bass guitar and drums) and muddiness in your sound.

On the other hand, if you set your filter too low, you can leave a gap between your subwoofer output and your speaker output.

Try this: initially set your low-pass filter at 80 Hz, then listen. If it doesn’t seem quite right, adjust the filter up and down until it sounds the way you want it to.

Some people use bass boosts to increase the output at certain low frequencies.

Another good feature to look for in a subwoofer amp is a subsonic filter. These work by blocking super-low frequencies that naturally occur in some songs. You can’t actually hear these frequencies, since they exist below the threshold of human hearing, but if they’re not blocked, your subs will waste energy trying to play them. By blocking them, you free up your sub to play the notes that you can hear more efficiently, which gives you a more satisfying experience.

Bass boosts

Many amps also offer bass boosts, which increase the output of your sub at targeted, specific frequencies. While some people like to engage boosts to get extra oomph in some songs, others tend to use them sparingly, if at all, in order to keep bass output in proportion with the rest of the system.

More questions? Call us

Bass is certainly a fun and exciting element of your music, but we understand that getting the best subwoofer setup in your vehicle can be a complex topic. Our advisors will be happy to walk you through the process, and help you find the stuff that’ll give you the experience you want.

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Crutchfield Writer Biography

Matt Freeman

A circuitous path, involving England, New York, rural Michigan, Indiana, and lots of parts in between brought Matthew Freeman to Charlottesville, where he's been writing about mobile audio/video for Crutchfield off and on since early 2000. He fosters an eclectic taste in film, and is fond of a wide range of music.
A native of Albuquerque, New Mexico, he found his way to the University of Notre Dame, where, in an act of charity unsurpassed in the history of Western civilization, he was given a B.A. in English.